1. Field of the Invention
The present invention relates generally to an apparatus and method for estimating interference and noise and more particularly to an apparatus and method for estimating a carrier to interference and noise ratio (CINR) that can estimate the CINR as one measure of reception performance in a wireless communication system.
2. Description of the Related Art
Recently, orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) has been used as a method for high-speed data transmission in a wired/wireless channel as a method for transmitting data using a plurality of sub-carriers. According to the OFDM/OFDMA, a serial to digital conversion operation is performed on input data, parallel data is modulated into a plurality of sub-carriers, that is, sub-channels, having inter-orthogonality, and the modulated parallel data is transmitted.
The OFDM is commonly applied to digital transmission technology such as digital/audio broadcasting, digital television (TV), a wireless local area network (WLAN), a wireless asynchronous transfer mode (WATM), a fixed or mobile broadband wireless access (BWA), etc.
Previously, the OFDM was not widely used because of hardware complexity, but the OFDM has recently become implementable with the development of various digital signal processing technologies including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). The OFDM is similar to conventional frequency division multiplexing (FDM) and can acquire optimum transmission efficiency during high-speed data transmission by transmitting sub-carriers while maintaining orthogonality there between. The OFDM has better frequency efficiency and is inherently robust against multi-path fading. The OFDM is robust against frequency selective fading using a superimposed frequency spectrum and reduces the effects of inter-symbol interference using a guard interval. Therefore, the OFDM enables simple design of a hardware equalizer and is robust against impulse noise.
The OFDM system may be required to measure a carrier to interference and noise ratio (CINR) as a parameter necessary for power control or adaptive modulation/and coding.
In the prior art relating to the OFDM system, there is “FAST AND ACCURATE SIGNAL-TO-NOISE RATIO ESTIMATION TECHNIQUE FOR OFDM SYSTEMS”, U.S. Pat. No. 6,456,653. U.S. Pat. No. 6,456,653 discloses a method for estimating a noise level from unused sub-carriers. The OFDM system performs an IFFT operation for data to be transmitted from a transmitter and transmits a result of the IFFT operation. When an IFFT size is based on N FFT points, N sub-carriers are used and N unused sub-carriers are filled with zeros. Among signals undergoing the FFT conversion of a receiver, the mixed data and noise are output from the N used sub-carriers, and only noise is output from the N unused sub-carriers. In U.S. Pat. No. 6,456,653, a noise level is measured from the N unused sub-carriers. It is assumed that a value of the measured noise level is the same as a level of noise mixed with data. The noise level is subtracted from a power level received from the N used sub-carriers, such that a true-signal level is estimated. As a result, a ratio between the true-signal level and the noise level is obtained as a desired SNR estimation value.
However, the conventional SNR estimation method suffers extreme degradation in estimation performance when the number of unused sub-carriers is very small as compared with the number of used sub-carriers. Further, the conventional SNR estimation method cannot estimate an interference signal, because the interference signal is not incoming into the unused sub-carriers from other users using the same band.